Data storage devices have continually been reduced in physical size while offering increased data storage capacity. One result of these competing goals is a general trend toward controlling the actuator motion by monitoring the voice coil motor Bemf rather than by reading servo information. For example, Bemf control is used to unlatch and park the head in the landing zone in a contact-start-stop type of data storage device. For another example, Bemf control is used when using an unloading ramp to park the heads while the device is not operational. In both examples, velocity control of the actuator cannot be provided by reading the servo information because the heads are moved away from the storage medium. In order to detect the Bemf, a detection circuit is typically provided with a sense resistor placed in series with the coil of the voice coil motor. However, the detection circuit must be able to cancel voltage drop contributions attributable to the voice coil motor and the sense resistor.
In previously attempted solutions the actuator is moved to a position that prevents further movement, such as abuttingly engaging and biased against a crash stop. This ensures that any energization of the voice coil motor toward the crash stop will result in a known zero velocity. In this physically restrained arrangement, two or more different levels of current can be applied to the voice coil motor, and the sense resistor gain value, Ksense, can be iteratively adjusted until the same Bemf output is obtained for the different current levels.
This attempted solution has many disadvantages. For example, iterative convergence schemes require indeterminate and usually intolerably long calibration times in view of the system controller's requirements. Other disadvantages are associated with the physical contacting engagement required between the actuator and the abutting member, such as the crash stop. Intentional contacting engagement of the actuator against the crash stop increases the risk of shock causing head/disc contact. To minimize this risk of damage, the actuator approach velocity is made small, but this likewise contributes to a slow calibration procedure. The intentional contacting engagement also increases the risk that particulates are created by the contacting bodies.
What is needed is a solution that performs real time calculations to non-iteratively derive the calibrated Ksense value without requiring the actuator be held stationary, such as by being biased against a crash stop. The needed solution would also provide better calibration accuracy by driving voice coil motor current in both directions rather than just one direction. The needed solution would also provide better predictability by providing a fixed-time calculation rather than an indeterminate iterative procedure. It is to these improvements that the embodiments of the present invention are directed.